Composition for silicone rubber foam, manufacturing method of silicone rubber foam, and silicone rubber foam

ABSTRACT

There are provided a composition for silicone rubber foam, a manufacturing method of a silicone rubber foam, and a silicone rubber foam in which it is possible to control the foaming states. The composition for silicone rubber foam contains: (A) 100 parts by mass of polyorganosiloxane having a degree of polymerization of 4,000 to 10,000 and having two or more alkenyl groups at a content of 0.001 mmol/g or more and less than 0.3 mmol/g; an amount of polyorganohydrogensiloxane having an average of two or more Si-atom-bonded hydrogen atoms so that a molar ratio of the hydrogen atoms to the alkenyl groups in the (A) component is 0.001 to 5; 0.1 to 10 parts by mass of an organic foaming agent with a decomposition temperature of 50 to 250° C.; 5 to 200 parts by mass of silica powder; and a catalyst amount of a platinum-based metal catalyst activated with ultraviolet rays.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior International ApplicationNo. PCT/JP2012/007402 filed on Nov. 19, 2012, which is based upon andclaims the benefit of priority from Japanese Patent Application No.2011-269565 filed on Dec. 9, 2011; the entire contents of all of whichare incorporated herein by reference.

FIELD

The present invention relates to a composition for silicone rubber foam,a manufacturing method of a silicone rubber foam, and a silicone rubberfoam.

BACKGROUND

The silicone rubber foam is well known as a material excellent inweather resistance, electric property, compression set and so on. Thesilicone rubber foam can be basically obtained by compounding an organicfoaming agent such as azobisisobutyronitrile (AIBN), azodicarbonamide(ADCA), dinitrosopentamethylenetetramine (DPT) or the like and a curingagent into a thermosetting millable silicone rubber composition, andthen foaming and curing the resultant by heating. In order to obtain asilicone rubber foam having excellent characteristics, it is necessaryto balance the foaming rate (decomposition rate) of the organic foamingagent and the curing rate of the silicone rubber composition.

More specifically, if the decomposition rate of the organic foamingagent is lower than the curing rate of the silicone rubber composition,cells become fine but the foaming ratio becomes low, whereas if thecuring rate of the silicone rubber composition is lower than thedecomposition rate of the organic foaming agent, gas generated bydecomposition of the organic foaming agent cannot be sufficientlycaptured, resulting in a low foaming ratio and an uneven cell structure.

In addition, since the organic foaming agent has a high decompositionrate and rapidly generates gas, it is necessary to timely bring thesilicone rubber composition into a predetermined cross-linked state whenthe organic foaming agent decomposes, in order to capture the generatedgas.

In the cross-linking by peroxide, the cross-linking rate can be adjustedto a certain degree by selection of an organic peroxide and acompounding amount, but it is difficult to strictly adjust thecross-linking density at a timing of decomposition of the organicfoaming agent, thus bringing about a disadvantage that it is required tocompound an excessive amount of organic foaming agent or to strictlymanage process conditions.

An addition-type thermosetting silicone rubber using a platinum catalyst(see Patent Reference 1 (JP-B 1972-043294 (KOKOKU))) or this siliconerubber in combination with the above-described cross-linking by peroxide(see Patent Reference 2 (JP-A 1977-081378 (KOKAI))) has a disadvantagethat the rubber is inferior in preservation stability when thecross-linking density at decomposition of the foaming agent is optimizedat a usual production rate and is inferior in productivity when priorityis given to the preservation stability because a long time is requiredto bring the rubber into a predetermined cross-linked state, though theadjustment of the cross-linking density at the timing of decompositionof the foaming agent is relatively easy.

Patent Reference 3 (JP-A 2010-047646 (KOKAI)) and Patent Reference 4(JP-A 2003-213132 (KOKAI)) disclose ultraviolet irradiation curablesilicone compositions using a platinum complex activated withultraviolet rays. However, the cured products obtained from thecompositions disclosed therein are rubber or gel.

SUMMARY

An object of the present invention is to provide a composition forsilicone rubber foam in which it is possible to arbitrarily control thecross-linked state of the silicone rubber during foaming to therebycontrol the foaming states such as the cell diameter, the foaming ratioand so on.

Another object of the present invention is to provide a manufacturingmethod of a silicone rubber foam in which it is possible to arbitrarilycontrol the cross-linked state of the silicone rubber during foaming tothereby control the foaming states such as the cell diameter, thefoaming ratio and so on, and to provide a silicone rubber foam obtainedby the method.

A composition for silicone rubber foam of the present inventioncontains:

-   -   (A) 100 parts by mass of alkenyl group-containing        polyorganosiloxane expressed by a following general formula        (A1), having a mass average degree of polymerization of 4,000 to        10,000 and having two or more R²s in the following general        formula (A1) as a content per mass in a molecule at a rate of        0.001 mmol/g or more and less than 0.3 mmol/g:

(where, R¹ represents an unsubstituted or substituted monovalenthydrocarbon group containing no aliphatic unsaturated group, R²represents an alkenyl group, and R³ represents a monovalent groupindicating each independent R¹ or R². In the formula (A1), n and mrepresent the total numbers of each recurring unit obtained by randompolymerization or block polymerization, and n+m+2 obtained by adding 2that is the number of terminal groups to n and m represents a massaverage degree of polymerization);

-   -   (B) an amount of polyorganohydrogensiloxane having an average of        two or more hydrogen atoms bonded to a silicon atom in a        molecule so that a molar ratio of the silicon-atom-bonded        hydrogen atoms to the alkenyl groups in the (A) component (the        number of moles of hydrogen atoms bonded to the silicon atoms in        the (B) component/the number of moles of the alkenyl groups in        the (A) component) is 0.001 to 5;    -   (C) 0.1 to 10 parts by mass of an organic foaming agent with a        decomposition temperature of 50 to 250° C.;    -   (D) 5 to 200 parts by mass of silica powder; and    -   (E) a catalyst amount of a platinum-based metal compound        catalyst activated with ultraviolet rays.

A manufacturing method of a silicone rubber foam of the presentinvention, includes: a step (1) of irradiating the aforementionedcomposition for silicone rubber foam of the present invention withultraviolet rays at a temperature lower than the decompositiontemperature of the (C) organic foaming agent to obtain a cross-linkedbody; and a step (2) of heat-treating the cross-linked body at atemperature equal to or higher than the decomposition temperature of the(C) organic foaming agent to obtain a silicone rubber foam. Further thepresent invention provides a silicone rubber foam obtained by theabove-described manufacturing method of the present invention.

According to the present invention, a composition for silicone rubberfoam can be provided in which it is possible to arbitrarily control thecross-linked state of the silicone rubber during foaming to therebycontrol the foaming states such as the cell diameter, the foaming ratioand so on.

According to the manufacturing method of the present invention, it ispossible to arbitrarily control the cross-linked state of the siliconerubber during foaming to thereby control the foaming states such as thecell diameter, the foaming ratio and so on of the silicone rubber foamto be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph indicating the relations between average celldiameters and foaming ratios of silicone rubber foams obtained inexamples.

DETAILED DESCRIPTION

Hereinafter, an embodiment of the present invention will be described.

[Composition for Silicone Rubber Foam]

A composition for silicone rubber foam of the present inventioncontains: (A) 100 parts by mass of alkenyl group-containingpolyorganosiloxane expressed by the above-described general formula(A1), having a mass average degree of polymerization of 4,000 to 10,000and having two or more R²s in the general formula (A1) as a content permass in a molecule at a rate of 0.001 mmol/g or more and less than 0.3mmol/g; (B) an amount of polyorganohydrogensiloxane having an average oftwo or more hydrogen atoms bonded to a silicon atom in a molecule sothat a molar ratio of the silicon-atom-bonded hydrogen atoms to thealkenyl groups in the (A) component (the number of moles of hydrogenatoms bonded to the silicon atoms in the (B) component/the number ofmoles of the alkenyl groups in the (A) component) is 0.001 to 5; (C) 0.1to 10 parts by mass of an organic foaming agent with a decompositiontemperature of 50 to 250° C.; (D) 5 to 200 parts by mass of silicapowder; and (E) a catalyst amount of a platinum-based metal compoundcatalyst activated with ultraviolet rays.

In the composition for silicone rubber foam of the present invention,the (A) component and the (B) component cross-link by addition reactionto form a cross-linked body of polyorganosiloxane. The (E) componentacting as a catalyst is a catalyst activated with ultraviolet rays toprovide excellent preservation stability. Further, the organic foamingagent being the (C) component acting by heat and the (E) componentacting with ultraviolet rays are used in combination to enable thecross-linking and the foaming to be controlled by separate operations,so that the control of the cell diameter and the foaming ratio of thesilicone rubber foam is easy.

Note that in this specification the “cross-linked body” obtained fromthe composition for silicone rubber foam of the present invention refersto a cross-linked body of polyorganosiloxane obtained by cross-linkingof the (A) component and the (B) component by addition reaction.Further, the silicone rubber refers to the cross-linked body ofpolyorganosiloxane in this specification.

Hereinafter, each component will be described.

((A) Component)

Alkenyl group-containing polyorganosiloxane of (A) is a base polymer ofthe composition of the present invention. The (A) component is asubstantially linear siloxane macromolecule expressed by the followinggeneral formula (A1):

In the formula (A1), R¹ represents an unsubstituted or substitutedmonovalent hydrocarbon group containing no aliphatic unsaturated group,R² represents an alkenyl group, and R³ represents a monovalent groupindicating each independent R¹ or R². In the formula (A1), n and mrepresent the total numbers of each recurring unit obtained by randompolymerization or block polymerization, and n+m+2 obtained by adding 2that is the number of terminal groups to n and m represents a massaverage degree of polymerization. Hereinafter, polyorganosiloxaneexpressed by the general formula (A1) is sometimes referred to aspolyorganosiloxane (A1).

Note that the above general formula (A1) does not always mean a blockcopolymer. More specifically, m representing the number of —R¹R²SiO— asintermediate units and n representing the number of —R¹ ₂SiO— asintermediate units do not represent the numbers in a block but representthe total numbers of the intermediate units existing in the wholemolecule respectively. In other words, polyorganosiloxane expressed bythe general formula (A1) may be a random copolymer. A siloxane skeletalstructure may include a few branches but is preferably linear because ofability of synthesizing polymer with high degree of polymerizationindicated below with good controllability.

The mass average degree of polymerization of polyorganosiloxane (A1),that is, the number of siloxane units is expressed by n+m+2 obtained byadding 2 that is the number of terminal groups to n and m in the generalformula (A1) and ranges from 4,000 to 10,000. The mass average degree ofpolymerization preferably ranges from 5,000 to 7,000. If the massaverage degree of polymerization of polyorganosiloxane (A1) is less than4,000, required operability cannot be obtained and sufficient mechanicalstrength cannot be obtained. On the other hand, if the mass averagedegree of polymerization of polyorganosiloxane (A1) exceeds 10,000,polymerization becomes difficult.

n and m are selected so that the requirement of the above-described massaverage degree of polymerization is satisfied and the content of R²s(including R²s in the terminal units represented as R³) per mass in amolecule becomes 0.001 mmol/g or more and less than 0.3 mmol/g. Thecontent of R²s preferably ranges from 0.005 to 0.1 mmol/g, and morepreferably 0.01 to 0.05 mmol/g. If the content of R²s is less than 0.001mmol/g, the strength of the cross-linked body is insufficient to fail toobtain a silicone rubber foam, and if the content of R²s is 0.3 mmol/gor more, the cross-linked body becomes fragile so that the strength ofobtained a silicone rubber foam decreases. Further, at least two R²sexist in one molecule because R²s perform addition reaction to form thecross-linked body.

R¹ is an unsubstituted or substituted monovalent hydrocarbon groupcontaining no aliphatic unsaturated group. Examples of R¹ includeunsubstituted hydrocarbon groups such as alkyl groups including a methylgroup, an ethyl group, a propyl group, a butyl group, a pentyl group, ahexyl group and so on; aryl groups including a phenyl group, a tolylgroup, a xylyl group and so on; aralkyl groups including a benzyl group,a phenethyl group and so on, and substituted hydrocarbon groups such asa chloromethyl group, a 3-chloropropyl group, a 3,3,3-trifluoropropylgroup, a 3-cyanopropyl group, a 3-methoxypropyl group and so on. Foreasiness of synthesis and handling of polyorganosiloxane (A1) andexcellent heat resistance, preferably 50% or more of R¹s are methylgroups, and particularly preferably all of R¹s are methyl groups. It ispossible to arbitrarily use a phenyl group especially when heatresistance, cold resistance, or radiation resistance is required, and touse a 3,3,3-trifluoropropyl group when oil resistance and/or solventresistance are/is required.

R² is an alkenyl group. Concrete examples of R² include a vinyl group,an allyl group, a 3-butenyl group, a 4-pentenyl group, a 5-hexenyl groupand so on. For easiness of synthesis and handling of polyorganosiloxane(A1) and easy addition reaction, R² is preferably a vinyl group. R²s maybe bonded to any silicon atoms in a molecule, but a part of R²s arepreferably bonded to silicon atoms at molecular terminals because theyexhibit excellent reactivity. As the (A) component, one kind may be usedor two or more kinds may be used in combination.

((B) Component)

The (B) component is polyorganohydrogensiloxane having a siloxaneskeletal structure in which a hydrogen atom and an organic group arebonded to a silicon atom and having an average of two or more hydrogenatoms in a molecule. When manufacturing a silicone rubber foam using thecomposition of the present invention, the (B) component forms across-linked body by reaction with the alkenyl group in the (A)component by the catalysis of the (E) component. An exemplified exampleof the organic group bonded to the silicon atom in the (B) component isan unsubstituted or substituted monovalent hydrocarbon group in the samerange with R¹ in the (A) component, and a methyl group is preferablebecause of easiness of synthesis and handling. The siloxane skeletalstructure of the (B) component may be any of linear-chain, branched, orcyclic.

The amount of hydrogen atoms bonded to silicon atoms in the (B)component as the content per mass is preferably 0.1 mmol/g or more and20 mmol/g or less. The hydrogen atoms may be bonded to any siliconatoms, but at least a part of them are preferably bonded to siliconatoms at molecular terminals because they exhibit excellent reactivity.The mass average degree of polymerization of the (B) component ispreferably 4 to 500, and more preferably 10 to 100 because the (B)component can come into contact with the (A) component while maintainingits liquid form at a reaction temperature of addition reaction. As the(B) component, one kind may be used or two or more kinds may be used incombination.

The content of the (B) component in the composition for silicone rubberfoam of the present invention is a sufficient amount for cross-linkingthe (A) component. The content of the (B) component is such an amountthat the molar ratio of the hydrogen atoms bonded to the silicon atomsin the (B) component to R²s (alkenyl groups) in the (A) component (thenumber of moles of hydrogen atoms bonded to silicon atoms in the (B)component/the number of moles of alkenyl groups in the (A) component) is0.001 to 5 because the (B) content can impart, to a silicone rubber foamto be obtained, a sufficient effect of cross-linking, more specifically,elasticity enough to form foam and from the viewpoint that the (B)component imparts, to a cross-linked body, a cross-linking densityexhibiting flexibility to facilitate foam formation by the action of theorganic foaming agent being the (C) component. Further, the above molarratio is preferably 0.01 to 3 in the composition of the presentinvention.

((C) Component)

The organic foaming agent being the (C) component is an organic foamingagent with a decomposition temperature of 50 to 250° C. The (C)component is compounded in the composition of the present invention anddecomposes by being heated in a cross-linked body obtained bycross-linking the (A) component and the (B) component to generate gas tothereby make the cross-linked body into foam. If the decompositiontemperature of the organic foaming agent is lower than 50° C., theorganic foaming agent is inferior in handling and preservationstability, and if exceeding 250° C., the organic foaming agent isinferior in productivity.

As the organic foaming agent, any organic foaming agent may be usedwithout any limitation as long as it is an organic foaming agent with adecomposition temperature of 50 to 250° C. conventionally used inmanufacture of a silicone rubber foam. Concrete examples of the organicfoaming agent include: azo-based compounds such asazobisisobutyronitrile (decomposition temperature: 102° C.),1,1′-azobis(1-acetoxy-1-phenylethane) (decomposition temperature: 106°C.), and azodicarbonamide (decomposition temperature: 205° C.); andnitroso compounds such as dinitrosopentamethylenetetramine(decomposition temperature: 203° C.), andN,N-dimethyl-N,N-dinitrosoterephthalamide (decomposition temperature:105° C.). The compounds decompose when heated at temperatures equal toor higher than the decomposition temperatures to generate N₂ or CO₂ gas.Further, it is possible to adjust foaming temperatures by using anadjuster such as urea or organic acid in combination with the organicfoaming agents.

Among them, organic foaming agents with decomposition temperatures of 80to 200° C. from the viewpoint of productivity and easiness of handlingare preferably used. Note that as the (C) component, one kind may beused or two or more kinds may be used in combination.

A compounding amount of the organic foaming agent being the component(C) in the composition of the present invention is 0.1 to 10 parts bymass per 100 parts by mass of the (A) component, and preferably 1 to 7parts by mass. If the compounding amount of the (C) component is lessthan 0.1 parts by mass, a sufficient amount of gas cannot be generatedby the decomposition to fail to obtain foam. On the other hand, if thecompounding amount of the (C) component exceeds 10 parts by mass, thefoam to be obtained is likely to deform to deteriorate in quality.

((D) Component)

A silica powder being the (D) component may be a publicly-known one thatis generally compounded in a silicone rubber. The (D) component has afunction of imparting appropriate fluidity to the composition beforecross-linking and imparting, to the cross-linked body ofpolyorganosiloxane obtained by cross-linking, a high mechanical strengthrequired according to the usage thereof.

The silica powder being the (D) component preferably has a specificsurface area measured by the BET method (hereinafter, referred to as aBET specific surface area) of 50 m²/g or more, more preferably 50 to 600m²/g, and much more preferably 100 to 400 m²/g to serve theabove-described function by being added to the composition of thepresent invention. The kind of silica is not particularly limited, butprecipitated silica, aerosol silica (fumed silica), pyrogenic silica orthe like is preferably used. In terms of reinforcing property, aerosolsilica is preferable.

The silica powder being the (D) component is absolutely essential as areinforcing filler of rubber but is likely to cause problems such asthickening, remarkable plasticization return and so on if the silicapowder is added as it is because many silanol groups (Si—OH groups)exist on the untreated silica surface. Therefore, it is preferable tosubject the surface of the silica powder to hydrophobic treatment. Thesurface treatment amount is preferably set to obtain a carbon amount onthe silica surface of 2.0 mass % or more, and more preferably 3.0 mass %or more. At the carbon amount of less than 2.0 mass %, the silica powderprovides less effect in suppression of the thickening of the compositionand improvement in pot life. Note that the upper limit of the carbonamount is not particularly limited, but is generally 20 mass % or less,preferably 12 mass % or less, and particularly preferably 8 mass % orless. As the silica powder being the (D) component, the one which hasbeen previously subjected to the surface treatment in a powder state maybe used, or the silica powder may be subjected to the surface treatmentin a kneading process.

As the surface treatment method of the silica powder, a generally knownsurface treatment technique can be employed. As examples of anorganosilicon compound used as a surface treatment agent includeorganosilazanes including hexaorganodisilazane such as1,3-divinyltetramethyldisilazane, 1,3-dimethyltetravinyldisilazane, andhexamethyldisilazane; and octaorganotrisilazane such asoctamethyltrisilazane, and 1,5-divinylhexamethyltrisilazane, silanecoupling agents including alkyltrialkoxysilane such asmethyltrimethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane,and butyltrimethoxysilane; dialkyldialkoxysilane such asdimethyldimethoxysilane, diethyldimethoxysilane, dimethyldiethoxysilane,and diethyldiethoxysilane; alkenyltrialkoxysilane such asvinyltriethoxysilane, vinyltrimethoxysilane andvinyltris(methoxyethoxy)silane; dialkenyldialkoxysilane such asdivinyldimethoxysilane and divinyldiethoxysilane; trialkylalkoxysilanesuch as trimethylmethoxysilane and triethylmethoxysilane;trialkenylalkoxysilane such as trivinylmethoxysilane andtrivinylethoxysilane; organochlorosilane such as trimethylchlorosilane,dimethyldichlorosilane, methyltrichlorosilane, vinyltrichlorosilane,divinyldichlorosilane, and trivinylchlorosilane;chloropropyltrimethoxysilane, dimethylpolysiloxane (including a cyclicstructure), organohydrogenpolysiloxane and the like, and may be apartial hydrolysis condensate thereof. Note that among them, asilane-based coupling agent having methyl groups as substituents bondedto the silicon atoms other than the hydrolyzable groups, cyclicdimethylpolysiloxane, and organosilazanes are preferable.

As the silica powder being the (D) component, a commercially-availableproduct may be used. As examples of the commercially-available productinclude Aerosil 200 (trade name, manufactured by EVONIC, BET specificsurface area: 200 m²/g), Aerosil 300 (trade name, manufactured byEVONIC, BET specific surface area: 300 m²/g) and the like as the aerosolsilica with the surface untreated. Further, in the present invention, itis preferable to use the silica powder made by subjecting thecommercially-available product to the surface treatment withoxtamethylcyclotetrasiloxane, hexamethyldisilazane or the like. As the(D) component, one kind may be used or two or more kinds may be used incombination.

A compounding amount of the (D) component in the composition of thepresent invention is 5 to 200 parts by mass per 100 parts by mass of the(A) component, and preferably 10 to 100 parts by mass. If thecompounding amount of the silica powder being the (D) component exceeds200 parts by mass, the viscosity of the composition significantlyincreases to degrade the workability in molding, whereas if thecompounding amount is less than 5 parts by mass, the characteristics ofthe mechanical strength and so on of the silicone rubber foam to beobtained are not sufficient.

((E) Component)

A platinum-based metal compound catalyst activated with ultraviolet raysbeing the (E) component catalyzes the addition reaction between the (A)component and the (B) component. The (E) component is a curing catalystthat is activated with ultraviolet irradiation and has ultravioletactivity of accelerating the addition reaction between the alkenylgroups in the (A) component and the hydrosilyl groups in the (B)component.

As the (E) component used in the present invention, any platinum-basedmetal compound may be used without any limitation as long as it acts asa catalyst with ultraviolet irradiation. Note that the platinum-basedmetal means platinum, rhodium, palladium and the like. The (E) componentis an ultraviolet-active catalyst designed as a compound containing aplatinum-based metal such that the catalyst activity appears by beingirradiated with ultraviolet rays. As the (E) component, concretely, acyclopentadienyl platinum compound is preferable, and(methylcyclopentadienyl)trimethylplatinum is particularly preferable. Asthe (E) component, one kind may be used or two or more kinds may be usedin combination.

A compounding amount of the (E) component in the composition of thepresent invention is an amount to catalyze the addition reaction betweenthe (A) component and the (B) component. Concretely, the compoundingamount of the (E) component is generally 0.1 to 1000 ppm, and preferably0.1 to 100 ppm in terms of platinum metal atoms to the total amount ofthe (A) component and the (B) component. The compounding amount of the(E) component in this range sufficiently catalyzes the addition reactionbetween the (A) component and the (B) component to obtain an excellentaddition reaction rate. To obtain excellent heat resistance in thesilicone rubber foam to be obtained, the compounding amount of the (E)component is more preferably 0.5 to 50 ppm, and particularly preferably1 to 8 ppm in terms of platinum metal atom.

(Optional Components)

The composition for silicone rubber foam of the present inventionpreferably further contains, as the (F) component, an organic peroxidewith a decomposition temperature equal to or higher than thedecomposition temperature of the (C) component. By compounding theorganic peroxide being the (F) component, the cross-linking furtherproceeds in the cross-linked body of polyorganosiloxane in operation offoaming performed at a temperature equal to or higher than thedecomposition temperature of the (C) component to form a silicone rubberfoam with excellent strength and high foaming ratio.

The organic peroxide being the (F) component is not particularly limitedas long as it has a decomposition temperature equal to or higher thanthe decomposition temperature of the (C) component and is generally usedin this kind of composition for silicone rubber foam. Concrete examplesof the organic peroxide include benzoyl peroxide, 2,4-dichlorobenzoylperoxide, p-chlorobenzoyl peroxide, dicumyl peroxide, 2,5-bis(t-butylperoxy)-2,5-dimethylhexane, 2,5-bis(t-butyl peroxy)-2,5-dimethylhexyne,di-t-butyl peroxide, t-butyl peroxy benzoate, bis(4-t-butylcyclohexyl)peroxydicarbonate and so on.

The organic peroxides are arbitrarily selected depending on thedecomposition temperature of the organic foaming agent being the (C)component to be used. The decomposition temperature of the organicperoxide is equal to or higher than the decomposition temperature of theorganic foaming agent being the (C) component to be combined, andpreferably higher by 10 to 60° C. than the decomposition temperature ofthe (C) component from the viewpoint of productivity. As the (F)component, one kind may be used or two or more kinds may be used incombination.

A compounding amount of the organic peroxide being (F) component in thecomposition of the present invention is preferably 0.001 to 10 parts bymass per 100 parts by mass of the (A) component, and more preferably 0.1to 5 parts by mass. If the compounding amount of the (F) component isless than 0.001 parts by mass, the addition effect of the organicperoxide cannot be sufficiently obtained, whereas if the compoundingamount exceeds 10 parts by mass, the characteristics such as heatresistance may be degraded in the silicone rubber foam to be obtained.

The composition for silicone rubber foam of the present invention maycontain a processing aid that is preferably added when compounding thesilica powder being the (D) component. As the processing aid,low-viscosity silicone oil, silicone resin, or silane compound havingsilanol groups or alkoxy groups at terminals is generally used. In thecomposition for silicone rubber foam of the present invention, it ispreferable to use polydimethylsiloxane having silanol groups at bothterminals with a mass average degree of polymerization of about 4 to 200as the processing aid.

A compounding amount of the processing aid in the composition of thepresent invention depends on the kind of the (D) component and thecompounding amount of the (D) component used with respect to the (A)component. The compounding amount of the processing aid is preferably0.1 to 20 parts by mass per 100 parts by mass of the (D) component, andmore preferably 1 to 10 parts by mass.

In the composition for silicone rubber foam of the present invention,various additives which have been conventionally used according tovarious kinds of purposes may be compounded, in addition to the abovecomponents, in a range not to inhibit the effects of the presentinvention. Examples of the additives include inorganic filler other thanthe silica powder being the (D) component, such as pulverized silica(quartz fine particle), diatomaceous earth, metal carbonate, clay, talc,mica, titanium oxide and the like, conductive filler such as a carbonblack, pigment, thixotropy imparting agent, viscosity adjuster forimproving the extrusion workability, ultraviolet ray protective agent,anti-mildew agent, heat resistance improver, flame retardant,antioxidant and the like.

The composition for silicone rubber foam of the present invention can beprepared by uniformly kneading the (A) component to the (E) component,and further the (F) component, the processing aid and other componentswhich are compounded as needed, by using a mixing device such as auniversal kneading machine, a kneader or the like.

Further, for example, an organic foaming agent master batch made bykneading the (C) component and a part of the (A) component involved infoaming, a catalyst master batch made by kneading the (E) component thatis the catalyst component for the addition reaction between the (A)component ant the (B) component and a part of the (A) component, and abase compound made by kneading the remainder of the (A) component, the(B) component, the (D) component, and the optional components notinvolved in the addition reaction between the (A) component and the (B)component, foaming and the like, such as the processing aid and so onmay be separately prepared and used by being mixed together immediatelybefore use. Note that in the case of using the (F) component, the (F)component is preferably mixed together with the master batches and thebase compound when they are mixed together immediately before use.

According to the composition for silicone rubber foam of the presentinvention, the foaming ratio can be designed in a wide range whilekeeping an excellent foaming state in the silicone rubber foam to beobtained by changing the ratio of the (B) component to change thecross-linking density of the silicone rubber while keeping constant theratio of the organic foaming agent being the (C) component to the (A)component in the above-described ranges. Further, the wide range offoaming ratio can be similarly designed also by changing the ratio ofthe (C) component while keeping constant the ratio of the (B) componentto the (A) component. Furthermore, by adjusting the amount of the (B)component with respect to the (A) component, the degree of cross-linkingcan be made suitable for the foaming, so that a high foaming ratio canbe obtained with a small addition amount of the organic foaming agentbeing the (C) component.

Manufacture of the silicone rubber foam using the composition forsilicone rubber foam of the present invention generally includes anultraviolet irradiation step of activating the catalyst being the (E)component for the addition reaction between the (A) component and the(B) component, and a heating step of decomposing the organic foamingagent being the (C) component to generate foaming gas. Whenmanufacturing the silicone rubber foam using the composition forsilicone rubber foam of the present invention, the ultravioletirradiation step and the heating step may be performed at the same time,but it is preferable to use the following manufacturing method of thepresent invention because the foaming ratio and the cell diameter of thesilicone rubber foam to be obtained can be sufficiently controlled.

Manufacturing Method of Silicone Rubber Foam

The manufacturing method of the silicone rubber foam of the presentinvention includes a step (1) of irradiating the composition forsilicone rubber foam of the present invention with ultraviolet rays at atemperature lower than the decomposition temperature of the (C) organicfoaming agent to obtain a cross-linked body, and a step (2) ofheat-treating the cross-linked body at a temperature equal to or higherthan the decomposition temperature of the (C) organic foaming agent toobtain a silicone rubber foam.

In the manufacturing method of the present invention, the compositionfor silicone rubber foam is generally molded into a shape to whichultraviolet irradiation in the step (1) is efficiently performed,concretely a sheet-shape, line shape, tube shape or the like before thestep (1. The thickness of the composition compact for silicone rubberfoam in the sheet shape or the like after molding is preferably about 1to 10 mm from the viewpoint that a cross-linked body in a uniformcross-linked state can be obtained by the ultraviolet irradiation andcells are thus uniformly formed by the foaming performed thereafter,that is, the uniformity of cells in the foam is secured. Examples of themethod of making the composition for silicone rubber foam into a compactin the sheet shape or the like with a uniform thickness include generalmolding methods of a composition for silicone rubber foam, such as anextrusion molding, a calendar molding and the like.

(Step (1))

In the present invention, in the step (1), to activate theplatinum-based metal compound catalyst activated with ultraviolet raysbeing the (E) component, preferably(methylcyclopentadienyl)trimethylplatinum, contained in the compositionfor silicone rubber foam, the composition for silicone rubber foam,preferably the above-described composition for silicone rubber foammolded in the sheet shape or the like is irradiated with ultravioletrays.

The wavelength of the irradiated ultraviolet rays can be generally 200nm to 400 nm, and preferably 250 nm to 400 nm depending on the kind ofthe (E) component used. The irradiation energy can be 100 mJ/cm² to100,000 mJ/cm² as an integrated light intensity at 365 nm. Note that anatmospheric temperature in irradiation of the ultraviolet rays is lowerthan the decomposition temperature of the (C) organic foaming agent.

As a light source used in the present invention only needs to be anapparatus that generates ultraviolet rays, and examples thereof include,for example, an ultra-high pressure mercury lamp, a xenon lamp, amercury-xenon lamp, a high pressure mercury lamp, a metal halide lamp,an intermediate pressure mercury lamp, a low pressure mercury lamp andso on. To irradiate the above-described composition for silicone rubberfoam molded into the sheet shape or the like with the ultraviolet raysuniformly also, in particular, in the thickness direction, a pluralityof the light sources may be combined.

In the manufacturing method of the present invention, the platinum-basedmetal compound catalyst activated with ultraviolet rays being the (E)component is activated to accelerate the addition reaction between the(A) component and the (B) component to obtain a cross-linked body ofpolyorganosiloxane in the step (1) as descried above.

(Step (2))

By heat-treating the cross-linked body obtained in the above at atemperature equal to or higher than the decomposition temperature of theorganic foaming agent being the (C) component, a silicone rubber foamcan be obtained. The temperature of the heat treatment is notparticularly limited as long as it is equal to or higher than thedecomposition temperature of the organic foaming agent, but ispreferably higher by 50 to 150° C. than the decomposition temperature.Incidentally, in the case of using an organic foaming agent with adecomposition temperature of 150 to 250° C., it is preferable to set theupper limit of the heat treatment temperature to 300° C. from theviewpoint of preventing thermal deterioration of the composition.Further, the period of the heat treatment is preferably set to 0.5 to 12hours. An example of the heat treatment method is a method of heattreatment in an oven or the like.

In the case where the composition for silicone rubber foam contains theorganic peroxide being the (F) component, further cross-linking iscarried out by the heat treatment. Accordingly, the temperature of theheat treatment in this case is preferably a temperature equal to orhigher than the decomposition temperature of the organic foaming agentbeing the (C) component and equal to or higher than the decompositiontemperature of the organic peroxide being the (F) component.Alternatively, the heat treatment in the step (2) may be performed at atemperature equal to or higher than the decomposition temperature of theorganic foaming agent being the (C) component and lower than thedecomposition temperature of the organic peroxide being the (F)component, and the obtained silicone rubber foam may be heat-treated ata temperature equal to or higher than the decomposition temperature ofthe organic peroxide being the (F) component to acceleratecross-linking.

According to the manufacturing method of the present invention, asilicone rubber foam with a foaming ratio of 1.1 to 6 times can beobtained. Note that the foaming ratio in this specification means afoaming ratio measured by a measurement method of foaming ratio used fora general silicone rubber foam. More specifically, the foaming ratio isa value obtained by using the density of an unfoamed silicone rubberobtained by cross-linking the composition for silicone rubber foam as areference and dividing the density of the unfoamed silicone rubber bythe density of the obtained silicone rubber foam. The silicone rubberfoam manufacture by the manufacturing method of the present inventionusing the composition for silicone rubber foam of the present inventionis a silicone rubber foam with wide range variation in foaming ratiowhile keeping an excellent foaming state.

Further, the silicone rubber foam manufactured by the manufacturingmethod of the present invention using the composition for siliconerubber foam of the present invention is a silicone rubber foam which isable to have cells formed relatively densely with respect to the foamingratio and can be adjusted in cell diameter in a wide range including asilicone rubber foam of a fine cell diameter. Specifically, a siliconerubber foam can be obtained in which an average cell diameter obtainedby measuring all of cell cross-sections existing per 1.65 mm² in anarbitrary cross-section of the silicone rubber foam using a microscopicimage is 10 to 700 μm. Note that the average cell diameter in this caseindicates the average value of the cell diameters measured in thevertical direction or the horizontal direction of the image about all ofthe cell cross-sections which are entirely captured in the microscopicimage indicating an area of 1.65 mm² of the cross-section of the foam.

EXAMPLES

Examples of the present invention will be described below but thepresent invention is not limited to those examples.

In examples and comparative examples, following polyorganosiloxane wasused as the (A) component, the (B) component and the processing aid(process oil). Note that siloxane unit is represented by the followingsymbols.

-   -   M unit: (CH₃)₃SiO_(1/2)—    -   M^(V) unit: (CH₃)₂(CH₂═CH)SiO_(1/2)—    -   M^(OH) unit: (CH₃)₂(OH)SiO_(1/2)—    -   D unit: —(CH₃)₂SiO—    -   D^(H) unit: —(CH₃)HSiO—    -   D^(V) unit: —(CH₃)(CH₂═CH)SiO—

(A) Component

Base polymer 1: linear polydimethylsiloxane with R¹s being methylgroups, both of two R³s being R²s, and R²s being vinyl groups in thegeneral formula (A1). Namely, linear polydimethylsiloxane containingvinyl groups at both terminals in which both terminals are capped withM^(V) units and intermediate units are composed of D units and D^(V)units. The mass average degree of polymerization is 7,000 and thecontent of vinyl groups (R²s) per mass in a molecule is 0.038 mmol/g.

Base polymer 2: linear polydimethylsiloxane with R¹s being methylgroups, both of two R³s being R²s, and R²s being vinyl groups in theabove general formula (A1). Namely, linear polydimethylsiloxanecontaining vinyl groups at both terminals in which both terminals arecapped with M^(V) units and intermediate units are composed of D unitsand D^(V) units. The mass average degree of polymerization is 4,000 andthe content of vinyl groups (R²s) per mass in a molecule is 0.02 mmol/g.

(B) Component

Polymethylhydrogensiloxane: linear polymethylhydrogensiloxane havingboth terminals capped with M units and having intermediate unitscomposed of D units and D^(H) units. The mass average degree ofpolymerization is 40 and the content of silicon-atom-bonded hydrogenatoms per mass in a molecule is 8.8 mmol/g.

Processing Aid (Process Oil)

Linear polydimethylsiloxane containing silanol groups at both terminals:linear polydimethylsiloxane containing silanol groups at both terminalsin which both terminals are capped with M^(OH) units and intermediateunits are composed of D units, having a mass average degree ofpolymerization of 50.

Further, in the example and the comparative examples, the followingcompounds, materials and so on were used as the (C) component to the (F)component.

(C) Component

1,1′-azobis(1-acetoxy-1-phenylethane): Otsuka Chemical Co., Ltd.,OT_(AZO)-15 (trade name) was used. Decomposition temperature was 106° C.

(D) Component

D4-treated Aerosil 200: aerosol silica having the surface treated withoctamethylcyclotetrasiloxane and a specific surface area of 200 m²/g.The carbon amount on the silica surface was 3.1 mass %. Aerosil 200(trade name, manufactured by EVONIC Co., Ltd.) was used as the aerosolsilica.

HMDZ-treated Aerosil 200: aerosol silica having the surface treated withhexamethyldisilazane and a specific surface area of 200 m²/g. The carbonamount on the silica surface was 4.1 mass %. Aerosil 200 (trade name,manufactured by EVONIC Co., Ltd.) was used as the aerosol silica.

(E) Component

(Methylcyclopentadienyl)trimethylplatinum: manufactured by StremChemicals, Inc. was used.

(F) Component

2,5-bis(t-butyl peroxy)-2,5-dimethylhexane (decomposition temperature of159° C.).

Examples 1 to 17 (a) Preparation of Composition for Silicone Rubber Foam

(a-1) Preparation of Master Batches of (C) Component (Organic FoamingAgent)

The (C) component master batches were prepared each by kneading1,1′-azobis(1-acetoxy-1-phenylethane) of parts by mass shown in Table 1and base polymer 1 (examples 1 to 15) or base polymer 2 (examples 16,17) of the same parts by mass as that of1,1′-azobis(1-acetoxy-1-phenylethane) for 1 hour by the kneader and thenpassing the resultant through 3-roll mill twice.

(a-2) Preparation of Master Batches of (E) Component (Platinum-BasedMetal Compound Catalyst Activated with Ultraviolet Rays)

The (E) component master batches were prepared each by kneading(methylcyclopentadienyl)trimethylplatinum of parts by mass shown inTable 1 and base polymer 1 (examples 1 to 15) or base polymer 2(examples 16, 17) in an amount of 99.87 parts by mass when the (E)component was 0.13 parts by mass for 1 hour by the kneader.

(a-3) Preparation of Base Compounds

In each of the examples 1 to 15, the base compound 1 was manufactured bykneading the remainder of the base polymer 1 used in the above (a-1) and(a-2) into D4-treated Aerosil 200 and processing aid (process oil:linear polydimethylsiloxane containing silanol groups at both terminals)in an amount both shown in Table 1, for 4 hours by the kneader.

In each of the examples 16, 17, the base compound 2 was prepared bykneading the remainder of the base polymer 2 used in the above (a-1) and(a-2) into HMDZ-treated Aerosil 200 in an amount shown in Table 1 for 4hours by the kneader.

(a-4) Preparation of Compositions for Silicone Rubber Foams Compositionsfor silicone rubber foams were obtained by mixing the base compoundsobtained in (a-3), the (c) component master batches obtained in (a-1)and the (E) component master batches obtained in (a-2), and othercomponents shown in Table 1 by a 2-roll mill into uniform compositions.

(b) Manufacture of Silicone Rubber Foams

(b-1) Ultraviolet Irradiation Step (Step (1))

The compositions for silicone rubber foams obtained in the above wereeach subjected to sheeting by a width of 100 mm and a thickness of 5 mmby using a 2-roll mill. The obtained compositions for silicone rubberfoams in the sheet shape were irradiated with ultraviolet rays (at awavelength of 365 nm, 1,000 mJ/cm²) from one side by a high pressuremercury lamp, whereby cross-linked bodies of polyorganosiloxane wereobtained.

(b-2) Heating Step (Step (2))

The cross-linked bodies obtained in (b-1) were heat-treated for fourhours in an oven at 200° C. to foam, whereby silicone rubber foams wereobtained. Note that in the compositions containing the (F) component,further cross-linking as well as foaming was performed by thistreatment.

[Evaluation]

The obtained silicone rubber foams were evaluated by the followingmethod. The results thereof are shown in Table 1 together with thecompositions of the compositions for silicone rubber foams.

(Foaming Ratio)

The density of the obtained silicone rubber foam was measured by thewater displacement method. The value was calculated as the foaming ratioby dividing the density of the unfoamed silicone rubber similarlymeasured in advance by the density of the obtained silicone rubber foam.

(Appearance Evaluation)

The foaming state at the surface and the cross-section of the siliconerubber foam was visually evaluated based on the following criteria.void: occurrence of void was confirmed at the surface. excellent: thesurface was smooth with uniform cells.

(Average Cell Diameter)

The cell diameters of all of the cell cross-sections which were entirelycaptured in the microscopic image per 1.65 mm² of the cross-sectionperpendicular to the main surface of the obtained silicone rubber foamwere measured in the vertical direction of the image, and the averagecell diameter [μm] was calculated.

TABLE 1 E1 E2 E3 E4 E5 E6 E7 E8 Composition (A) Base polymer 1 100 100100 100 100 100 100 100 of Base polymer 2 — — — — — — — — Composition(B) Polymethylhydrogensiloxane 0.90 0.45 0.22 0.11 0.06 0.03 0.01 0.11(parts by (C) 1,1′-azobis (1- 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29mass) acetoxy-1-phenylethane) (D) D4-treated Aerosil 200 33.39 33.3933.39 33.39 33.39 33.39 33.39 33.39 HMDZ-treated Aerosil 200 — — — — — —— — (E) (methylcyclopentadienyl) 0.0017 0.0017 0.0017 0.0017 0.00170.0017 0.0017 0.0017 trimethylplatinum (F) 2,5-bis (t-butyl 0.99 0.990.99 0.99 0.99 0.99 0.99 — peroxy)-2,5-dimethylhexane Linearpolydimethylsiloxane containing 2.86 2.86 2.86 2.86 2.86 2.86 2.86 2.86silanol groups at both terminals Si-bonded H in (B) component/Vinylgroups 2.07 1.04 0.52 0.26 0.13 0.06 0.03 0.26 in (A) component (molarratios) Ratio of (C) component to (A) + (B) 8 8 8 8 8 8 8 8 (ppm interms of Pt) Manufacture Ultraviolet irradiation PD PD PD PD PD PD PD PDEvaluation Foaming ratio 1.19 1.35 1.80 2.40 2.95 3.71 3.83 1.49 Surfacestate (appearance ELT ELT ELT ELT ELT ELT ELT ELT evaluation) Averagecell diameter (μm) — — — 125 325 415 433 95 E1 to E8 = Example 1 toExample 8; PD = Performed; ELT = Excellent E9 E10 E11 E12 E13Composition (A) Base polymer 1 100 100 100 100 100 of Base polymer 2 — —— — — Composition (B) Polymethylhydrogensiloxane 0.01 0.01 0.03 0.030.06 (parts by (C) 1,1′-azobis (1- 1.61 6.37 1.61 6.37 2.20 mass)acetoxy-1-phenylethane) (D) D4-treated Aerosil 200 33.97 32.32 33.9732.32 33.77 HMDZ-treated Aerosil 200 — — — — — (E)(methylcyclopentadienyl) 0.0017 0.0017 0.0017 0.0017 0.0017trimethylplatinum (F) 2,5-bis (t-butyl 1.00 0.96 1.00 0.96 1.00peroxy)-2,5-dimethylhexane Linear polydimethylsiloxane containing 2.912.77 2.91 2.77 2.89 silanol groups at both terminals Si-bonded H in (B)component/Vinyl groups 0.03 0.03 0.07 0.06 0.13 in (A) component (molarratios) Ratio of (C) component to (A) + (B) 8 8 8 8 8 (ppm in terms ofPt) Manufacture Ultraviolet irradiation PD PD PD PD PD EvaluationFoaming ratio 2.40 4.26 2.35 4.42 2.45 Surface state (appearance ELT ELTELT ELT ELT evaluation) Average cell diameter (μm) 169 476 161 — 161 E14E15 E16 E17 Composition (A) Base polymer 1 100 100 — — of Base polymer 2— — 100 100 Composition (B) Polymethylhydrogensiloxane 0.05 0.11 0.110.06 (parts by (C) 1,1′-azobis (1- 6.37 6.37 3.36 3.36 mass)acetoxy-1-phenylethane) (D) D4-treated Aerosil 200 32.32 32.32 — —HMDZ-treated Aerosil 200 — — 38.39 38.39 (E) (methylcyclopentadienyl)0.0017 0.0017 0.0009 0.0009 trimethylplatinum (F) 2,5-bis (t-butyl 0.960.96 1.01 1.01 peroxy)-2,5-dimethylhexane Linear polydimethylsiloxanecontaining 2.77 2.77 — — silanol groups at both terminals Si-bonded H in(B) component/Vinyl groups 0.13 0.25 0.50 0.25 in (A) component (molarratios) Ratio of (C) component to (A) + (B) 8 8 4 4 (ppm in terms of Pt)Manufacture Ultraviolet irradiation PD PD PD PD Evaluation Foaming ratio5.23 4.42 3.48 3.71 Surface state (appearance ELT ELT ELT ELTevaluation) Average cell diameter (μm) 668 — — — E9 to E17 = Example 9to Example 17; PD = Performed; ELT = Excellent

Comparative Examples 1 to 8

The compositions for silicone rubber foams with compositions shown inTable 2 were obtained similarly to the examples. Further, the siliconerubber foams were manufactured similarly to the examples except that theultraviolet irradiation step of (b-1) in the above examples wasperformed or not performed as shown in Table 2, and then evaluated. Theresults thereof are shown in Table 2.

TABLE 2 CE1 CE2 CE3 CE4 CE5 CE6 CE7 CE8 Composition (A) Base polymer 1100 100 100 100 100 100 100 — of Base polymer 2 — — — — — — — 100Composition (B) Polymethylhydrogensiloxane — — 0.11 — 0.11 — — — (partsby (C) 1,1′-azobis (1-acetoxyy-1-phenylethane) 3.29 3.29 3.33 3.33 3.331.61 6.37 3.36 mass) (D) D4-treated Aerosil 200 33.39 33.39 33.83 33.8333.83 33.97 32.32 — HMDZ-treated Aerosil 200 — — — — — — 38.39 (E)(methylcyclopentadienyl) 0.0017 0.0017 — — — 0.0017 0.0017 0.0009trimethylplatinum (F) 2,5-bis (t-butyl 0.99 0.99 1.00 1.00 1.00 1.000.96 1.01 peroxy)-2,5-dimethylhexane Linear polydimethylsiloxanecontaining 2.86 2.86 2.90 2.90 2.90 2.91 2.77 — silanol groups at bothterminals Si-bonded H in (B) component/Vinyl groups in (A) — — 0.26 —0.26 — — — component (molar ratios) Ratio of (C) component to (A) + (B)(ppm in terms of Pt) 8 8 — — — 8 8 4 Manufacture Ultraviolet irradiationPD NPD NPD PD PD PD PD PD Evaluation Foaming ratio 2.25 2.40 2.25 2.092.09 1.76 2.56 2.09 Surface state (appearance evaluation) Void Void VoidVoid Void ELT Void ELT Average cell diameter (μm) 314 407 299 254 226216 — — CE1 to CE8 = Comparative Example 1 to Comparative Example 8; PD= Performed; NPD = Not Performed; ELT = Excellent

As is seen from Table 1, in the composition for silicone rubber foam ofthe present invention, the foaming ratio can be designed in a wide rangeby changing the ratio of the (B) component to change the cross-linkingdensity of the silicone rubber while keeping constant the ratio of theorganic foaming agent being the (C) component to the (A) component.

Further, the wide range of foaming ratio can be similarly designed alsoby changing the ratio of the (C) component while keeping constant theratio of the (B) component to the (A) component. In this case, thefoaming ratio is higher as the ratio of the (C) component is higher.

On the other hand, in the compositions for silicone rubber foams in thecomparative examples 1 to 5, and the comparative example 7, the surfacestate of the foam is deteriorated because the composition does notcontain the (B) component or the (E) component and thus cannot obtainthe cross-linked body. The comparative example 6 has the samecomposition as that of the example 9 except that it does not contain the(B) component, but can obtain only a low foaming ratio as compared tothe example 9 because it cannot efficiently take the gas generated fromthe (C) component into the foam due to insufficient control of thecross-linked state, and its cell state is coarse as compared with theexample 9. Similarly, the comparative example 8 has the same compositionas those of the examples 16, 17 except that it does not contain the (B)component, but can obtain only a low foaming ratio due to insufficientcontrol of the cross-linked state.

Further, the relations between the average cell diameters [μm] and thefoaming ratios of the silicone rubber foams obtained in the aboveexamples and comparative examples are plotted on a graph as FIG. 1. Notethat the comparative examples with void are also plotted on the graph.As is seen from FIG. 1, comparing the average cell diameters at similarfoaming ratios, the examples controlled in cross-linking during foamingbecame foams with fine cell diameter as compared with the comparativeexamples. On the other hand, when comparing regarding the additiveamount of the same organic foaming agent (C), the examples controlled incross-linking during foaming efficiently captured gas generated from thefoaming agent and became foams with high foaming ratio as compared withthe comparative examples.

It is found that, according to the present invention, a silicone rubberfoam having an excellent foaming state even at a high foaming ratio canbe obtained, and a silicone rubber foam with relatively small averagecell diameter with respect to the foaming ratio can be obtained.

According to the composition for silicone rubber foam of the presentinvention, a silicone rubber foam can be provided in which it ispossible to arbitrarily control the cross-linked state of the siliconerubber during foaming to thereby control the foaming states such as thecell diameter, the foaming ratio and so on are controlled. According tothe manufacturing method of the present invention, it is possible toarbitrarily control the cross-linked state of the silicone rubber duringfoaming to thereby control the foaming states such as the cell diameter,the foaming ratio and so on of the silicone rubber foam to be obtained.The composition for silicone rubber foam of the present invention isparticularly suitable for an extrusion mold and a calendar mold, and thesilicone rubber foam to be obtained is preferably used as a gasket, ashock-absorbing material, a heat insulating material and so on.

What is claimed is:
 1. A composition for silicone rubber foamcomprising: (A) 100 parts by mass of alkenyl group-containingpolyorganosiloxane expressed by a following general formula (A1), havinga mass average degree of polymerization of 4,000 to 10,000 and havingtwo or more R²s in the following general formula (A1) as a content permass in a molecule at a rate of 0.001 mmol/g or more and less than 0.3mmol/g,

where R¹ represents an unsubstituted or substituted monovalenthydrocarbon group containing no aliphatic unsaturated group, R²represents an alkenyl group, R³ represents a monovalent group indicatingeach independent R¹ or R², n and m represent the total numbers of eachrecurring unit obtained by random polymerization or blockpolymerization, and n+m+2 obtained by adding 2 that is the number ofterminal groups to n and m represents a mass average degree ofpolymerization; (B) an amount of polyorganohydrogensiloxane having anaverage of two or more hydrogen atoms bonded to a silicon atom in amolecule so that a molar ratio of the silicon-atom-bonded hydrogen atomsto the alkenyl groups in the (A) component (the number of moles ofhydrogen atoms bonded to the silicon atoms in the (B) component/thenumber of moles of the alkenyl groups in the (A) component) is 0.001 to0.26; (C) 0.1 to 10 parts by mass of an organic foaming agent with adecomposition temperature of 50 to 250° C.; (D) 5 to 200 parts by massof silica powder; and (E) a catalyst amount of a platinum-based metalcompound catalyst activated with ultraviolet rays.
 2. The compositionfor silicone rubber foam according to claim 1, further comprising (F)0.001 to 10 parts by mass, per 100 parts by mass of the (A) component,of organic peroxide with a decomposition temperature equal to or higherthan the decomposition temperature of the (C) component.
 3. Thecomposition for silicone rubber foam according to claim 1, wherein 0.1to 1000 ppm of (methylcyclopentadienyl)trimethylplatinum in terms ofplatinum metal atoms with respect to the total amount of the (A)component and the (B) component is contained as the (E) platinum-basedmetal compound catalyst.
 4. The composition for silicone rubber foamaccording to claim 2, wherein 0.1 to 1000 ppm of(methylcyclopentadienyl)trimethylplatinum in terms of platinum metalatoms with respect to the total amount of the (A) component and the (B)component is contained as the (E) platinum-based metal compoundcatalyst.